Camptothecin (CPT), shown in formula I:
was isolated and purified by Wall and coworkers in 1966 (J. Am. Chem. Soc. 88, 3888, (1966)). This compound was initially tested against the mouse leukemia L 1210 system and found to be active. The compound was quickly tested in human clinical trials. However, the subsequent biological evaluation indicated that this compound is highly toxic and consequently is unusable as a chemotherapeutic agent (Gottlieb et al., Cancer Chemother. Rep. 54, 461, (1970), and 56, 103, (1972); Muggia et al., Cancer Chemother. Rep. 56, 515, (1972); Moertel et al., Cancer Chemother. Rep. 56, 95, (1972); and Schaeppi et al., Cancer Chemother. Rep. 5, 25, (1974)). The reason for the failure of the early trial was later found to be due to the selection of an incorrect drug formulation. Camptothecin is insoluble in water. In order to use the drug for intravenous (iv) administration, camptothecin was converted to its sodium form (CPT sodium carboxylate). This form, although water-soluble, is practically devoid of anticancer activity. For example, a careful evaluation of these agents in animal models made by Wani et al. revealed that the sodium salt is only 10-20% as potent as the parent camptothecin (J. Med. Chem. 23, 554, (1980)). Important parameters for the anticancer activity of camptothecin derivatives have now been established (Wall et al., Ann. Rev. Pharmacol. Toxicol. 17, 117, (1977)). The intact lactone form with an α-hydroxyl group with the (S)-configuration at the C-20 position of the molecule is essential for antitumor activity. Maintaining the molecule as an intact lactone is critical for successful treatment.
Camptothecin and camptothecin derivatives are cytotoxic compounds which can be used as chemotherapeutic agents. The cytotoxic activity of camptothecin compounds is believed to arise from the ability of these compounds to inhibit both DNA and RNA synthesis and to cause reversible fragmentation of DNA in mammalian cells. Camptothecin compounds inhibit the enzyme DNA topoisomerase I which is known to relax supercoiled DNA. This relaxation is brought about by breakage of one of the DNA strands in the formation of a covalent topoisomerase I-DNA complex. Camptothecin derivatives are believed to function by reversibly trapping the enzyme-DNA intermediate which is termed the “cleavable complex.” (Hsiang et al. Cancer Research, 49, 4385, (1989)). The cleavable complex assay developed by Hsiang et al. is a standard test for determining the cytotoxic activity of camptothecin compounds.
Camptothecin and its derivatives have shown a spectacular activity against a wide spectrum of human tumors grown in xenografts in nude mice (Giovanella et al., Cancer Res. 51, 3052, (1991), and Natelson et al., Annals N.Y. Acad. Sci. 803, 224, (1996)), but much less activity was observed in human clinical trials. This difference in antitumor activity has been associated with the finding that the hydrolysis of lactone to carboxylate of the molecule is much faster in human plasma than in mouse. Burke and coworkers have systematically studied the stability of camptothecin derivatives in human serum (Annals N.Y. Acad. Sci. 803, 29, (1996)).
Ten-hydroxycamptothecin (10-HCPT) is a derivative of camptothecin, and also a natural occurring compound. This compound was obtained as an accompanying product on isolation of camptothecin and can now be synthesized from camptothecin in a number of ways. Currently, two anti-cancer agents directly derived from 10-HCPT are commercially available for treatment. One is topotecan, and the other is irinotecan; and their structures are as follows:

The molecule 10-HCPT, is very potent against cancer cells. Unfortunately, this molecule is not useful for cancer treatment because of its toxicity. The molecule bears two hydroxyl groups, one each at the C-10 and C-20 positions. The C-20 hydroxyl group is adjacent to the carbonyl group of the E-ring of the molecule, which constructs a reactive α-hydroxy lactone moiety. This feature of the molecule makes the lactone moiety very sensitive to hydrolysis, and thus, the molecule is not stable in the body. The 10-phenolic hydroxyl group of 10-HCPT is not stable in the process of enzymatic metabolism reactions. It is well known that phenolic hydroxy-containing moiety of an organic compound can be enzymatically oxidized into a semi-quinone or quinone during the process of metabolism. The corresponding semi-quinone or quinone metabolite is usually more toxic than the parental phenolic compound.
Thus, there is a need for protected 10-HCPT derivatives which are stable in the body and which have a longer biological life span.
Several reports have disclosed the esterification at the C-20 position of camptothecin derivatives. U.S. Pat. Nos. 5,968,943 and 6,407,239, respectively, disclose the preparation of alkyl and aromatic ester products of camptothecins by introduction of an acyl group at the C-20 position. U.S. Pat. Nos. 6,040,313 and 4,943,579 disclose the esterification of the hydroxyl group at the C-20 position of camptothecin compounds produces a non-toxic water-soluble prodrug. The prodrug is non-toxic even though the parent camptothecin compound itself may be substantially more toxic. Hydrolysis of the ester formed at the C-20 position reforms the parent camptothecin compound after administration, thereby reducing the overall toxicity experienced by the patient during camptothecin therapy.
U.S. Pat. No. 6,492,335 describes the glycoconjugates of camptothecin derivatives in which at least one carbohydrate component is linked via suitable spacers with the 20-hydroxyl group of a camptothecin derivative. U.S. Pat. No. 6,376,617 reported the preparation of water soluble polymeric conjugates of camptothecin
with N-(2-hydroxypropyl)methacryloylamide linked via a spacer group to the C-20 position. The conjugates possess enhanced antitumor activity and decreased toxicity with respect to the free drug. U.S. Pat. No. 5,646,159 discloses the esterification of 10,11-dioxymethylenecamptothecin with amino acid derivatives as acylating reagents at the C-20 position to provide several water-soluble compounds. U.S. Pat. No. 5,731,316 discloses the preparation of alkyl or alkenyl ester products of camptothecins by the esterification reaction at the C-20 position.
These patents disclose the single protection of the CPT molecule, meaning that the esterification reaction takes place at the C-20 position. Although 10-HCPT and its derivatives are very potent against cancer cells, they are not very useful due to high toxicities, lack of stability, and shortened biological life spans. Thus, it is very desirable to use the present invention which is able to overcome the problems associated with prior art 10-HCPT and other camptothecin derivatives.
Microparticles and foreign bodies present in the blood are generally cleared from the circulation by the “blood filtering organs”, namely the spleen, lungs and liver. The particulate matter contained in normal whole blood comprises red blood cells (typically 8 microns in diameter), white blood cells (typically 6-8 microns in diameter), and platelets (typically 1-3 microns in diameter). The microcirculation in most organs and tissues allows the free passage of these blood cells. When microthrombii (blood clots), with the size greater than 10-15 microns, are present in circulation, a risk of infarction or blockage of the capillaries will be generated, leading to ischemia or oxygen deprivation and possible tissue death. Injection into the circulation of particles greater than 10-15 microns in diameter, therefore, must be avoided. A suspension of particles less than 7-8 microns, is however, relatively safe and has been used for the delivery of pharmacologically active agents in the form of liposomes and emulsions, nutritional agents, and contrast media for imaging applications.
The size of particles and their mode of delivery determine their biological behavior. Strand et al. (in Microspheres-Biomedical Applications, ed. A. Rembaum, pp 193-227, CRC Press (1988)) have described the fate of particles to be dependent on their size. Particles in the size range of a few nanometers (nm) to 100 nm enter the lymphatic capillaries following interstitial injection, and phagocytosis may occur within the lymph nodes. After intravenous/intraarterial injection, particles less than about 2 microns will be rapidly cleared from the blood stream by the reticuloendothelial system (RES), also known as the mononuclear phagocyte system (MPS). Particles larger than about 7 microns will, after intravenous injection, be trapped in the lung capillaries. After intraarterial injection, particles are trapped in the first capillary bed reached. Inhaled particles are trapped by the alveolar macrophages.
Intravenous drug delivery permits rapid and direct equilibration with the blood stream which carries the medication to the rest of the body. To avoid the peak serum levels which are achieved within a short time after intravascular injection, administration of drugs carried within stable carriers would allow gradual release of the drugs inside the intravascular compartment following a bolus intravenous injection of the therapeutic nanoparticles.
Injectable controlled-release nanoparticles can provide a pre-programmed duration of action, ranging from days to weeks to months from a single injection. They also can offer several profound advantages over conventionally administered medicaments, including automatic assured patient compliance with the dose regimen, as well as drug targeting to specific tissues or organs (Tice and Gilley, Journal of Controlled Release 2, 343-352 (1985)).
Pharmaceuticals that are water-insoluble or poorly water-soluble and sensitive to acid environments in the stomach cannot be conventionally administered (e.g., by intravenous injection or oral administration). The parenteral administration of such pharmaceuticals has been achieved by emulsification of the oil solubilized drug with an aqueous liquid (such as normal saline) in the presence of surfactants or emulsion stabilizers to produce stable microemulsions. These emulsions may be injected intravenously, provided the components of the emulsion are pharmacologically inert. U.S. Pat. No. 4,073,943 describes the administration of water-insoluble pharmacologically active agents dissolved in oils and emulsified with water in the presence of surfactants such as egg phosphatides, pluronics (copolymers of polypropylene glycol and polyethylene glycol), polyglycerol oleate, etc. PCT International Publication No. W085/00011 describes pharmaceutical microdroplets of an anaesthetic coated with a phospholipid such as dimyristoyl phosphatidylcholine having suitable dimensions for intradermal or intravenous injection.